An unusual peptide conformation may precipitate amyloid formation in Alzheimer's disease: application of solid-state NMR to the determination of protein secondary structure.
The formation of insoluble proteinaceous deposits is characteristic of many diseases which are collectively known as amyloidosis. There is very little molecular-level structural information available regarding the amyloid deposits due to the fact that the constituent proteins are insoluble and noncrystalline. Therefore, traditional protein structure determination methods such as solution NMR and X-ray crystallography are not applicable. We report herein the application of the solid-state NMR technique rotational resonance (R2) to the accurate measurement of carbon-to-carbon distances in the amyloid formed from a synthetic fragment (H2N-LeuMetValGlyGlyValValIleAla-CO2H) of the amyloid-forming protein of Alzheimer's disease (AD). This sequence has been implicated in the initiation of amyloid formation. Two distances measured by R2 indicate that an unusual structure, probably involving a cis amide bond, is present in the aggregated peptide amyloid. This structure is incompatible with the accepted models of fibril structure. A relationship between this structure and the stability of the amyloid is proposed.